Periodic or random vibrations or shock can excite the resonant frequencies in circuit articles such as printed circuit boards, printed wire boards, flexible circuits, etc. For example, excited resonant frequencies in a circuit board can be problematic due to the resultant formation of undesirable stresses, displacements, fatigue, mechanical forces, and even sound radiation. Such undesirable vibrations or shocks are typically induced by external forces and can be experienced by a wide variety of circuit articles and under a variety of conditions. For example, printed circuit boards (PCBs) have the electrical traces laid-out upon or within a base substrate or laminate and typically have various components such as integrated circuit chips, resistors, capacitors, and the like, placed on and connected to these various electrical traces or other connection devices. Resonant vibrations can cause problems in circuit articles such as printed circuit boards and cards, etc., by significantly increasing the mechanical displacement of the base substrate or laminate, which may result in undesirable stresses and fatigue and subsequent premature circuit article failure. In addition, components which extend away from the surface of the circuit article can experience significant mechanical displacement due to the resonant vibrations in the circuit article thus potentially leading to mechanical damage of the components, premature failure of connection points and lower part reliability.
Various techniques have been used to reduce vibrational and shock amplitude effects on circuit articles, such as PCBs, with limited success.
A first known method involves the addition of flat, non-metallic, dampers to the exterior surface of the circuit article, i.e., "add-on dampers". These add-on dampers, utilize viscoelastic materials in exterior surface damping treatments for vibration and shock control. Two types of exterior surface damping treatments are commonly used: (a) free layer exterior damping treatment; and (b) constrained layer exterior damping treatment. Both of these exterior damping treatments can provide high levels of damping to a structure, i.e., dissipation of undesirable vibrations. Examples of such damping techniques are described, for example, in U.S. Pat. Nos. 2,819,032 (issued Jan. 7, 1953); 3,071,217 (issued Jan. 1, 1963); 3,078,969 (issued Feb. 26, 1963); 3,159,249 (issued Dec. 1, 1964); and 3,160,549 (issued Dec. 8, 1964).
Free layer damping treatment is also referred to as "unconstrained layer" or "extensional damping" treatment. In this technique, damping occurs by applying a layer of viscoelastic damping material to an external surface of a structure. The material can be applied to one or both sides of a structure. The mechanism by which this treatment dissipates undesirable energy (e.g., resonant vibrations), involves deformation. That is, when the structure is subjected to cyclic loading, for example, the damping material is subjected to tension-compression deformation and dissipates the energy through an extensional strain mechanism.
Another add-on damping treatment is constrained layer damping treatment. Constrained layer damping treatment is also referred to as "shear damping" treatment. For a given weight, this type of damping treatment is generally more efficient than the free layer damping treatment. In this technique, damping occurs by applying a damper of one or more layers of viscoelastic damping material and one or more flat layers of a higher tensile modulus material which is free of projections to the external surface of a structure. That is, this damping technique is similar to the free layer damping treatment wherein a viscoelastic material is applied to an exposed surface of a structure, the difference being that the viscoelastic material is additionally constrained by a flat projection-free layer having a higher modulus than the viscoelastic material, e.g., a metal layer, in the constrained layer treatment. Energy dissipates from the viscoelastic damping material via a shear mechanism that results from constraints by the higher modulus constraining layer and the base structure.
The problems associated with these dampers are numerous and have greatly limited the use of these dampers for PCB applications. These dampers typically cover a significant surface area of the PCBs (typically about 60-100% of the surface area on one side of the PCB to achieve significant damping benefits) and typically cover numerous short components or features less than 0.04 inch (1.02 mm) in height!, such as small integrated circuits, small resistors, circuit traces/lines, on the PCBs as a result. The coverage needed means that many PCBs cannot use dampers of this design as they do not have the needed "open" surface area for attachment of components or may require a costly damper design with cut-outs, holes, etc. These dampers may also require a thick damping polymer layer(s) to allow coverage of circuit components and to achieve good damping thus causing cost and application concerns. These damping treatments can be difficult to remove for rework, as removal can potentially cause damage to the PCB. As these dampers cover a large surface area, the ability to do failure analysis and rework in areas covered by the dampers is not directly possible.
In addition, these dampers can retain significant amounts of heat in the PCB, leading to premature component failures. The damping material could potentially be corrosive to components on the PCB and since such a large surface area is covered, it is difficult to isolate and leave uncovered components. These dampers tend to be large which can be costly in terms of raw materials and manufacturing, plus the dampers can add significant weight to the PCB. Non-metallic constraining layers are used since so many components come into contact or near contact with the damper and thus could short.
Although these exterior surface damping techniques are used, the degree of damping is oftentimes limited by maximum allowable thickness or other spacing requirements, obstructions on the circuit article surface which limits available damper coverage area, article accessibility, and/or environmental limitations. For example, if the circuit article is desired to be a component in a computer server design and thus has its surface nearly fully utilized by coverage of circuit components, the surface area needed to have an effective constrained layer damper is limited. In order for a known constrained layer damper to be effective, a large surface area of the article to be damped typically needs to be covered by the conventional constrained layer damper to be most effective. Typically 60-90% of the surface area of one side of the circuit board needs to be covered to most effectively damp the first 1-7 vibration mode shapes of the circuit board. The total circuit board surface area includes the upper surface, lower surface, and side(s). Some circuit boards have constrained layer dampers attached to both major sides of the circuit board to achieve acceptable damping.
For a circuit board in which one side has less components than the other, thus enabling the use of a constrained layer damper, the damper typically adds mass that is near the mass of the PCB base laminate itself, thus potentially doubling the weight of the PCB. The nearly full coverage constrained layer damper covers a large surface area and thus may significantly reduce the heat radiation, conduction or convection loss of the PCB leading to a hotter running PCB thus potentially reducing PCB component life or requiring added heat loss methods for the PCB or server design. The full coverage constrained layer damper also can be difficult to remove after application, rendering rework and/or inspection of the items covered by the by the damper impossible or impractical. As mentioned previously, the full size constrained layer damper may also need to cover some low profile components &lt;40 mils (1.02 mm)!, such as a small resistors, small capacitors, etc. thus leading to a damper design requiring a very thick damping material to allow sufficient conformance to the PCB, increasing damping material cost.
A second known method of damping circuit articles includes isolation of the circuit article or the structure in which it is used. Vibration is controlled by isolating the circuit article from the vibration source. U.S. Pat. No. 4,053,943 (issued Oct. 11, 1977) describes a system for externally isolating a printed circuit board. Laminated damping elements are remotely positioned from and connected to the circuit board by post structural means fastened at each end to the circuit board and to the laminated damping element. Such an external isolation system adds to the overall size of the circuit board and may be impractical where close positioning of the circuit board to other structures is desired. Another isolation method is described in WO 92/21178. By increasing the natural frequency of the circuit board, isolation can be achieved, again requiring complex design.
Isolation of a circuit article within a structure may not always be possible due to overall thickness requirements, space requirements, and/or the number of components on the circuit article surface. In addition, some damping may be required in conjunction with isolation to obtain the desired damping results, particularly at low frequencies.
Another method to improve the vibration resistance of a circuit article is to attach an aluminum stiffener(s) as shown in FIG. 35. These stiffener(s) serve to stiffen the circuit article and make the circuit article more difficult to vibrate. The stiffener(s) also serve to change the natural frequencies of the circuit article. The stiffener in FIG. 35, which is discussed in more detail in Comparative Example 4, is a long thin rectangular piece of aluminum having two major opposed surfaces and having legs which extend from one side of the stiffener. The stiffener(s) is attached by soldering to the circuit article via the legs. Stiffeners are added to the circuit article until the desired results are achieved in terms of increased stiffness.
However, the stiffened design may also require attachment to an external structure to be most effective. The stiffened PCB can also be very difficult to rework due to the stiffeners being soldered to the PCB. The stiffeners can also make rework of the PCB time consuming and costly. The stiffening effect for a PCB also may have limited benefits for improved PCB reliability or vibration resistance, as the vibration energy is shifted around the PCB by stiffeners and is not significantly dissipated. Thus, other components may have increased vibration problems on the PCB (tall integrated circuits, tall capacitors, resistors or connectors, for example) as the stiffener has not reduced significantly the total vibrational energy reaching and effecting the PCB.
Circuit articles such as printed circuit boards may contain numerous components. Some of these components may be more massive than in the past. Examples include integrated circuit chips, heat sinks, etc. In addition, larger circuit boards are becoming more common increasing the likelihood of sagging or deflection of the PCB during dynamic or static inputs (i.e., during assembly or as an assembled part in a design) leading to a reliability problem with respect to the circuit boards' line traces, displacement of components and the likelihood of a more difficult installation of the PCB into a device, shelf, or rack due to sagging or vibrations. In addition, integrated circuit chips which are mounted via clips, pins, clamps, locks, pressure fits, channels, etc. are becoming increasingly popular. Compared to conventional integrated circuit chips which are mounted via adhesives, soldering etc., these removable chips are more susceptible to shock or vibration that can lead to small movements or micromotions that can create wear between components or surfaces, create heat, increase electrical resistance between components and can lead to poor electrical performance and/or chip connection failure and reliability problems.
Stiffening the PCB to make the PCB vibration resistant by increasing the natural frequencies of the PCB above the resonant frequencies that could be excited in the PCB by external forces such as by increasing the thickness of the PCB can be very expensive. Making the PCB thicker to increase stiffness adds material costs, can render the PCB non-standard for thickness, and can render processing longer and more complex. Furthermore, vibration energy may not be dissipated as desired.
The use of these vibration and shock control methods can add significant cost and/or complexity to the structure in which the circuit article is incorporated In addition their use is often limited by space considerations on and around the PCB surfaces, and also rework requirements. Moreover, failure analysis needs can be significantly reduced, the "package size" of the PCB can be significantly increased, costly manufacturing processes involving soldering on the PCB may be required, and a corrosion potential may need to be addressed for some designs. Other potential concerns include thermal conduction, thermal conductivity or thermal radiation changes caused by using the known dampers and stiffeners, in addition to the potential for electrical performance changes in the circuit article.